Transportation and the Urban Form
Author: Dr. Jean-Paul Rodrigue
1. Global Urbanization
No discussion about the urban spatial structure can take
place without an overview of urbanization, which has been
one of the dominant trends of economic and social change
of the 20th century, especially in the
developing world.

Urbanization. The process of transition from a rural
to a more urban society. Statistically, urbanization reflects
an increasing proportion of the population living in settlements
defined as urban, primarily through net rural to urban migration.
The level of urbanization is the percentage of the total
population living in towns and cities while the rate of
urbanization is the rate at which it grows (UNFPA, 2007).

This transition is expected to go on well into the second
half of the 21st century, a trend reflected in the growing
size of cities and in
the increasing proportion of the
urbanized population. Cities also dominate the
national
economic output as they account for the bulk of the production,
distribution and consumption. Urban mobility problems have
increased proportionally, and in some cases exponentially,
with urbanization since mobility demands are concentrated
over a specific area. Since 1950, the world's urban
population has more than doubled, to reach nearly 3.5 billion
in 2010, about 50.6% of the global population. This is the
outcome of three main demographic trends:

Natural increase. It is simply the outcome of
more births than deaths in urban areas, a direct function
of the fertility rate as well as the quality of healthcare
systems (lower mortality rates, particularly for infants).
Phases in the demographic
transition are commonly linked with urbanization rates.
Although this factor played an important role in the past,
it is of much lesser importance today as fertility rates
in many developed countries have dropped significantly,
in some cases like Western Europe, Japan and South Korea
below replacement rate.

Rural to urban migrations. This has been a strong
factor of urbanization, particularly in the developing world
where migration accounted between 40 and 60% of the urban
growth. Such a process has endured since the beginning of
the industrial revolution in the 19th century, first in
the developed world and then in the developing world. The
reasons for urban migration are numerous and may involve
the expectation to find employment, improved agricultural
productivity which frees rural labor or even political and
environmental problems where populations are constrained
to leave the countryside.

International migration. The growth
in international migration has been an important factor
in the urbanization of major gateway cities, such as Los
Angeles, Miami, New York, London and Paris. This process
has a tendency to take place in the largest cities, but
there is a trickle down to cities of smaller size.

Fundamental changes in the socio-economic environment of
human activities as urbanization involves new forms of employment,
economic activity and lifestyle have been observed. Thus,
industrialization in the developing world is directly correlated
with urbanization, the case of China being particularly
eloquent. The industrialization of coastal China has led
to the largest rural to urban migration in history. According
to the United Nations Population Fund, about 18 million
people migrate from rural areas to cities each year in China
alone.
Current global trends indicate a growth of about 50 million
urbanites each year, roughly a million a week. More than
90% of that growth occurs in developing countries which
places intense pressures on urban infrastructures, particularly
transportation, to cope (see Concept
4). By 2050, 6.4 billion people, about two thirds of
humanity, are likely to be urban residents. What can be
considered as urban includes a whole
continuum of urban spatial
structures, ranging from small towns to large urban
agglomerations. This also brings the question about optimal
city size since technical limitations (road, utilities)
are not much an impediment in building very large cities.
Many of the world's largest
cities can be labeled as dysfunctional mainly because
as city size increases the rising complexities are not effectively
coped with managerial expertise.
2. The Urban Form
Demographic and mobility growth have been shaped by the
capacity and requirements of urban transport infrastructures,
such as roads, transit systems or simply walkways. Consequently,
there is a wide variety of urban forms, spatial structures
and associated urban transportation systems.

Urban form. Refers to the
spatial imprint of an urban
transport system as well as the adjacent physical infrastructures.
Jointly, they confer a level of spatial arrangement to cities.

Urban (spatial) structure. Refers to the set of relationships
arising out of the urban form and its underlying interactions
of people, freight and information. It tries to evaluate
to what extent specific urban structures can be achieved
with specific transport systems.

In light of transport developments, the urban spatial structure
can be
categorized by its level of centralization and clustering:

Centralization. Refers to the setting
of activities in relation to the whole urban area. A centralized
city has a significant share of its activities in its center
while a decentralized city does not. Large employers such
as financial institutions are the main drivers of centralization.

Clustering. Refers to the setting of
activities in relation to a specific part of the urban area.
A cluster of activities is therefore a concentration around
a specific focal point, which tend to be transport infrastructures
such as a highway interchange, a transit terminal or a smaller
town that has been absorbed by the expansion of the metropolis.

Nodes. These are reflected in the centrality
of urban activities, which can be related to the spatial
accumulation of economic activities or to the accessibility
to the transport system. Terminals, such as ports, train
station, railyards, and airports, are important nodes around
which activities agglomerate at the local or regional level.
Nodes have a hierarchy related to their importance and contribution
to urban functions, with high order nodes such as management
and retailing and lower order nodes such as production and
distribution.

Linkages. These are the infrastructures supporting
flows from, to and between nodes. The lowest level of linkages
includes streets, which are the defining elements of the
urban spatial structure. There is a hierarchy of linkages
moving up to regional roads and railways and international
connections by air and maritime transport systems.

Depending on their nature, urban nodes and linkages provide
for a functional connectivity,
implying interdependent urban functions related to trade,
production and telecommunications. Urban transportation
is thus associated with a spatial form which
varies according to the
modes being used. What has not changed much is that
cities tend to opt for a grid street pattern. This
was the case for many Roman cities built in the 1st century
as it was for American cities built in the 20th century.
The reasons behind this permanence are relatively simple;
a grid pattern jointly optimizes accessibility and available
real estate. Obviously, many cities are not organized as
a grid. They correspond to older cities, many former fortified
towns, as well as cities which grew from a constrained location
such as an island or a river junction. Local geographical
and historical characteristics remain important influences
on the urban form.
In an age of motorization and personal mobility, an increasing
number of cities are developing a spatial structure that
increases reliance on motorized transportation, particularly
the privately owned automobile. This has incited a
shift from a grid pattern
towards curvilinear and cul-de-sac patterns that are
commonly found in suburban areas. Dispersion, or
urban sprawl, is taking place in many different types of
cities, from dense, centralized European metropolises such
as Madrid, Paris, and London, to rapidly industrializing
metropolises such as Seoul, Shanghai, and Buenos Aires,
to those experiencing recent, fast and uncontrolled urban
growth, such as Mumbai and Lagos. Recent urban expansion
is consequently almost all geared towards the automobile.
Therefore, there are significant
differences in
the density of cities across the world, in addition
to a variety of density
gradients are observed within cities. The differences are particularly prevalent between
North American and European cities.
3. Evolution of Transportation and Urban Form
Historically, movements within cities tended to be restricted
to walking, which made medium and long distance urban linkages
rather inefficient and time-consuming. Thus, activity nodes
tended to be agglomerated and urban forms compact.
Many modern cities have inherited an urban form created
under such circumstances, even though they are no longer
prevailing. The dense urban cores of many European, Japanese
and Chinese cities, for example, enable residents to make
between one third and two thirds of all trips by walking
and cycling. At the other end of the spectrum, the dispersed
urban forms of most Australian, Canadian and American cities,
which were built recently, encourages automobile dependency
and are linked with high levels of mobility. Many major
cities are also port cities with maritime accessibility
playing an enduring role not only for the economic vitality
but also in the urban spatial structure with the port district
being an important node. Airports terminals have also been
playing a growing role in the urban spatial structure as
they can be considered as cities within cities.
The evolution of transportation has generally led to
changes in urban form.
The more radical the changes in transport technology have
been, the more the alterations on the urban form. Among
the most fundamental changes in the urban form is the emergence
of new clusters expressing new
urban activities and new relationships between elements
of the urban system. In many cities, the central business
district (CBD), once the primary destination of commuters
and serviced by public transportation, has been changed
by new manufacturing, retailing and management practices.
Whereas traditional manufacturing depended on centralized
workplaces and transportation, technological and transportation
developments rendered modern industry more flexible. In
many cases, manufacturing relocated in a suburban setting,
if not altogether to entirely new low cost locations offshore.
Retail and office activities are also suburbanizing, producing
changes in the urban form. Concomitantly, many important
transport terminals, namely port facilities and railyards,
have emerged in suburban areas following new requirements
in modern freight distribution brought in part by containerization.
The urban spatial structure shifted from a nodal to a multi-nodal
character, implying new forms of urban development and new
connections to regional and global economic processes.
Initially, suburban growth mainly took place adjacent to
major road corridors, leaving plots of vacant or farm land
in between. Later, intermediate spaces were gradually filled
up, more or less coherently. Highways and ring roads, which
circled and radiated from cities, favored the development
of suburbs and the emergence of important sub-centers that
compete with the central business district for the attraction
of economic activities. As a result, many new job opportunities
have shifted to the suburbs (if not to entirely new locations
abroad) and the activity system of cities has been considerably
modified. Different parts of a city have different dynamism
depending on its spatial pattern. These changes have occurred
according to a variety of geographical and historical contexts,
notably in North America and
Europe as each subsequent phase of urban transportation
developments led to different spatial structures. Sometimes,
particularly when
new modern
urban road infrastructures are built, the subsequent
changes in the urban form can be significant.
Two processes had a substantial impact on contemporary urban
forms:

Dispersed urban land development patterns have
been dominant in North America since the end of World War
II, where land is abundant, transportation costs were low,
and where the economy became dominated by tertiary and quaternary
activities. Under such circumstances, it is not surprising
to find that there is a strong
relationship between urban
density and automobile use. For many cities their built
up areas have grown at a faster rate than their populations.
In addition, commuting became relatively inexpensive compared
with land costs, so households had an incentive to buy lower-priced
housing at the urban periphery. Similar patterns can be
found in many European cities, but this change is occurring
at a slower pace and involving a smaller range.

The decentralization of activities resulted in
two opposite effects. First, commuting time has remained
relatively stable in duration. Second, commuting increasingly
tends to be longer and made by using the automobile rather
than by public transit. Most transit and road systems were
developed to facilitate suburb-to-city, rather than suburb-to-suburb,
commuting. As a result, suburban highways are often as congested
as urban highways.

Although transportation systems and travel patterns have
changed considerably over time, one enduring feature remains
that most people travel between
30-40 minutes in one direction. Globally, people are
spending about 1.2 hours per day commuting, wherever this
takes place in a low or a high mobility setting. Different
transport technologies, however, are associated with
different travel speeds and capacity.
As a result, cities that rely primarily on non-motorized
transport tend to be different than auto-dependent cities.
Transport technology thus plays a very important role in
defining urban form and the spatial pattern of various activities.
Still, the evolution of the urban form is path dependent,
implying that the current spatial structure is obviously
the outcome of past developments, but that those developments
were strongly related to local conditions involving to the
setting, physical constraints and investments in infrastructures
and modes.
4. The Spatiality of Urban Transportation
The amount of urban land allocated to transportation is
often correlated with the level of mobility. In the pre-automobile
era, about 10% of the urban land was devoted to transportation
which was simply roads for a dominantly pedestrian traffic.
As the mobility of people and freight increased, a growing
share of urban areas was allocated to transport and the
infrastructures supporting it. Large variations in the spatial
imprint of urban transportation are observed between different
cities as well as between different parts of a city, such
as between central and peripheral areas. The major components
of the spatial imprint of urban transportation are:

Pedestrian areas. Refer to the amount of space
devoted to walking. This space is often shared with roads
as sidewalks may use between 10% and 20% of a road's
right of way. In central areas, pedestrian areas tend to
use a greater share of the right of way and in some instances
whole areas are reserved for pedestrians. However, in a
motorized context, most pedestrian areas are for servicing
people's access to transport modes such as parked automobiles.

Roads and parking areas. Refer to the amount
of space devoted to road transportation, which has two states
of activity; moving or parked. In a motorized city, on average
30% of the surface is devoted to roads while another 20%
is required for off-street parking. This implies for each
car about 2 off-street and 2 on-street parking spaces. In
North American cities, roads and parking lots account between
30 and 60% of the total surface.

Cycling areas. In a disorganized form, cycling
simply shares access to pedestrian and road space. However,
many attempts have been made to
create spaces specifically for
bicycles in urban areas, with reserved lanes and parking
facilities. The
Netherlands
has been particularly proactive over this issue with biking
paths parts of the urban transport system; 27% of the total
amount of commuting is accounted by cycling.

Transit systems. Many transit systems, such as
buses and tramways, share road space with automobiles, which
often impairs their respective efficiency. Attempts to mitigate
congestion have resulted in the creation of road lanes reserved
to buses either on a permanent or temporary (during rush
hour) basis. Other transport systems such as subways and
rail have their own infrastructures and, consequently, their
own rights of way.

Transport terminals. Refer to the amount of space
devoted to terminal facilities such as ports, airports,
transit stations, railyards and distribution centers. Globalization
has increased the mobility of people and freight, both in
relative and absolute terms, and consequently the amount
of urban space required to support those activities. Many
major terminals are located in the peripheral areas of cities,
which are the only locations where sufficient amounts of
land are available.

The spatial importance of each transport mode varies according
to a number of factors, density being the most important.
If density is considered as a gradient,
rings of mobility represent
variations in the spatial importance of each mode at providing
urban mobility. Further, each transport mode has
unique performance and space
consumption characteristics. The most relevant example
is the automobile. It requires space to move around (roads)
but it also spends 98% of its existence stationary in a
parking space. Consequently, a significant amount of urban
space must be allocated to accommodate the automobile, especially
when it does not move and is thus economically and socially
useless. In large urban agglomerations close to all the
available street parking space in areas of average density
and above is occupied throughout the day. At an aggregate
level, measures reveal a significant
spatial imprint of road transportation among developed
countries. In the United States, more land is thus used
by the automobile than for housing. In Western Europe, roads
account for between 15% and 20% of the urban surface while
for developing countries this figure is about 10% (6% on
average for Chinese cities but growing fast due to motorization).
5. Transportation and the Urban Structure
Urbanization involves an increased numbers of trips in urban
areas. Cities have traditionally responded to growth in
mobility by expanding the transportation supply,
by building new highways and transit lines. In the developed
world, that has mainly meant building more roads to accommodate
an ever-growing number of vehicles. Several urban spatial
structures have accordingly emerged, with the reliance
on the automobile being the most important discriminatory
factor. Four major types can be identified at the metropolitan
scale:

Type II - Weak Center: Representing
the spatial structure of many American cities where many
activities are located in the periphery.

Type III - Strong Center: Representing
high density urban centers with well developed public transit
systems, particularly in Europe and Asia.

Type IV - Traffic Limitation.
Representing urban areas that have implemented traffic control
and modal preference in their spatial structure. Commonly,
the central area is dominated by public transit.

There are different scales
where transportation systems influence the structure of
communities, districts and the whole metropolitan area.
For instance, one of the most significant impacts of transportation
on the urban structure has been the clustering of activities
near areas of high accessibility. The impact of transport
on the spatial structure is particularly evident in the
emergence of suburbia. Although many other factors are important
in the development of suburbia, including low land costs,
available land (large lots), the environment (clean and
quiet), safety, and car-oriented services (shopping malls),
the spatial imprint of the automobile is dominant. Even
if it could be argued that roads and the automobile have
limited impacts on the extent of urban sprawl, they are
a required condition for sprawl to take place. While it
is difficult to assess in which specific circumstances the
first suburbs emerged, suburban developments have occurred
in many cities worldwide, although no other places have
achieved such a low density and automobile dependency than
in the North America. The automobile is also linked with
changes in street layouts. While older parts of cities tend
to have a conventional grid layout, from the 1930s new suburbs
started to be designed in a curvilinear fashion, which included
some cul-de-sacs (dead ends). By the 1950s, the prevailing
design for new suburbs was privileging cul-de-sacs. Although
the aim was to create a more private and safe environment,
particularly in cul-de-sac sections, the outcome was also
a growing sense of isolation and car use.
Facing the expansion of urban areas, congestion problems
and the increasing importance of inter-urban movements the
existing structure of urban roads was judged to be inadequate.
Several ring roads have been
built around major cities and became an important attribute
of the spatial structures of cities, notably in North America.
Highway interchanges in suburban
areas are notable examples of clusters of urban development
that have shaped the multicentric character of many cities.
The extension (and the over-extension) of urban areas have
created what may be called peri-urban areas. They
are located well outside the urban core and the suburbs,
but are within reasonable commuting distances; the term "edge
cities" has been used to label a cluster of urban development
taking place in a suburban settings.

MediaGlobalization
and UrbanizationThe
Economic Output of the World's Major Metropolitan Areas
World Urban Population, 1950-2010 with Projections to 2020
Demographic Transition
Cities with more than 10 million Inhabitants
World's Largest Cities, 1850 World's
Largest Cities, 2010 World
at Night Urban Population
(in 1,000s) and Percentage of Urban Population per Continent,
1950-2030
Perspectives about the Urban Spatial Structure: From Dichotomy
to Continuum Transportation,
Urban Form and Spatial Structure
Types of Urban Spatial Structures Nodes,
Links and Urban Form Cities
and Connectivity One Hour
Commuting According to Different Urban Transportation Modes Evolution of
the Spatial Structure of a City
Urban Transport Development Paths
Street Network Types
High Density Structured Urban Form, Paris
Possible Urban Movement Patterns
Evolution of Transportation and Urban Form in North American
and European Cities
Population Density of the World's Largest Metropolitan
Areas Evolution
of Urban Densities in North America and Europe
Access Ramp to the Nanpu Bridge, Shanghai
Population Density by Distance from City Center, Selected
Cities
Population Density Changes by Census Block, Chicago 2000-2010 Density
and Car Use in North American Cities, 1991
Average Commuting Time (One Way), Selected Metropolitan
Areas
Service Attributes of Urban Transport Modes Pedestrian,
Cycling and Road Spaces, Amsterdam, Netherlands
Share of Cycling over the Total Amount of Trips, mid 1990s
Rings of Mobility Performance
of Urban Transport Modes Land Area
Consumed by the Car in Selected Countries, 1999 The Rationale
of a Ring Road Type I - Completely
Motorized Network Type II - Weak
Center Type III - Strong
Center Type IV - Traffic
Limitation Scale
and Urban Spatial Structure Suburban
Development along an Highway Interchange